Scanning system for light tracking device



Y Filed Dec. 20. 1960 July 6, 1965 A. H. RosENTHAL 3,192,824

l SCANNING SYSTEM FOR LIGHT TRACKING DEVICE 2 Sheets-Sheet 2 l-ELIC".

P19/51s a; JG

United States Patent O 3,192,824 SCANNING SYSTEM FCR LIGHT TRACKINGDEVICE Adolph H. Rosenthal, Forest Hills, N.Y., assigner to KollsmanInstrument Corporation, Elmhurst, NX., a corporation of New York FiledDec. 20, 1960, Ser. No. 77,199 i 4 Claims. (Cl. 88--1) My inventionrelates to a novel scanning system for light tracking devices wherein asingle scanning means can be used for scanning an image along two axesperpendicular to one another and to the optical axis of the system.

Scanning systems for light tracking devices are applicable to systems ofthe type shown in United States Patent 2,905,828 to OMaley et al.,assigned to the assignee of the present invention.

Light tracking devices of the type set forth in the above noted patentare utilized primarily for navigational purposes and are provided withan optical system adapted to transmit an image of a celestial objectsuch as a star, the sun or .the moon to a means which will seek tooperate the optical system to maintain the image in the center of thefield of view. The movements of the optical system may then betranslated into corresponding movements of operating or adjustingmembers for vehicle guidance instruments or devices.

The background of the field of view is frequently illuminated inconjunction with the celestial body to be tracked. Thus, a scanningmeans is desired for scanning .the image of the celestial object in amanner to minimize errors caused by background lighting.

Typical scanning devices of the type to which the present invention isapplicable are shown in copending applications Serial No. 47,837, filedAugust 8, 1960, entitled Light Modulation System, and Serial No. 77,198,now abandoned, filed December 20, 1960, entitled Scanning Device forLight Tracking Systems, and Serial No.

71,248, filed November 23, 1960, entitled Tuning Fork Scanner, all ofwhich are assigned to the assignee of the present invention.

In the above noted applications, devices using two independent scanningdevices which will scan an azimuth axis and altitude axis respectively`are provided to obtain information of altitude and azimuth of atelescope with relation to a celestial body. Thus, in application SerialNo. 47,837, a rst vibrating reed carries a first vaperture plate, whilea second vibrating reed carries a second aperture plate. The reeds areso arranged and driven that the apertures therein will define a scanningaxis first in a first direction and thereafter in a second andperpendicular direction to establish both an altitude and azimuth axisrespectively.

Alternative to the use of a vibrating reed driven, for example, by asolenoid, the Iaperture plates could also be mounted on one tine ofrespective tuning forks which are electromagnetically driven in themanner disclosed in copending application Serial No. 71,248.

A third manner in which the aperture plates could be arranged is setforth in copending application Serial No. 77,198, now abandoned, whereinthe image of .the celestial object is split into two components whichare directed to respective vibrating aperture systems wherein both anazimuth axis and altitude axis can be simultaneously established ratherthan first establishing one and then the other ina synchronous manner.

In accordance with the present invention, I provide means whereby only asingle scanning means need be provided, although both azimuth scanningand altitude scanning are achieved with the single device.- Morespecifically, in the present invention I cause the image of thecelestial object to rotate by .at predetermined intervals whereby asingle reed scanner positioned in the focal plane of the telescopeobjective will sean the image in an azimuth direction and, with therotation of the image by 90, will thereafter scan in an altitudedirection.

In order to rotate the image as described above, a Dove prism may bepositioned in front of the telescope objective whereby a rotation of theprism by 45 will cause the image of the telescope objective to rotate by90. Clearly, any other equivalent optical rotating device can be used.

Since with the novel invention, only a single scanning means is requiredfor scanning two axes of the image, it will be .apparent that I havesubstantially increased the reliability of the system in reducing .thenumber of oscillating mechanical parts required. Further, the inventionmakes possible a decrease in the weight of the system where the weightof the optical means for rotating the image is less than the weight of acomplete scanning mechanism.

As another embodiment of the invention, a lightweight obscuring diskhaving an opening .therein in front of the telescope objective can berotated to permit incident light to pass through a first or second Doveprism which are positioned at 45 with respect to one another. Thus, .theimage passed by one prism is rotated by 90 with respect to the imagepassed by the other prism, and only a simple lightweight disk need berotated.

Accordingly, a primary object of this invention is to provide a novelscanning system for light tracking devices.

Another object of this invention is to provide a single scanning devicefor a light tracking system which can develop an azimuth axis and analtitude axis which are independent of one another.

Another object of this invention is to provide alternate azimuth andaltitude tracking for a light tracking device by providing means forrotating the image of the body being tracked.

A further object of this invention is to provide a single scanning meansfor a light tracking device wherein the image of the body being trackedis rotatable by 90 so that the single scanning device will scan inazimuth or altitude depending upon the angular position of the image.

These and other objects of my invention will become apparent from thefollowing description when taken in connection with the drawings, inwhich:

FIGURE l shows a side cross-sectional view of a typical star trackingdevice which can utilize a single scanning mechanism for scanning in twoaxes, in accordance with the present invention.

FIGURE 2 shows .a side cross-sectional view of the novel image rotatingmechanism of FIGURE 1 taken across the lines 2-2 of FIGURE 1.

FIGURE 3 shows a top view of a reed-type scanner which may be used inthe tracking device of FIGURE 1.

FIGURE 4 is a side cross-sectional view of the reed mechanism of FIGURE3.

FIGURE 5 is a side cross-sectional view of a portion of the telescope ofFIGURE 1 which shows a second embodiment of an image rotating meanscomprised of two right angle mirrors. g

FIGURE 6 is a top view of FIGURE 5.

FIGURE 7 is a side view of FIGURE 5.

FIGURE 8 illustrates the mechanism of FIGURE 5 when one of the mirrorsis rotated to rotate the image passed by the telescope objective.

FIGURE 9 schematically illustrates the operation of the embodiment shownin FIGURES 5 through 8.

FIGURE 10 is a side cross-sectional view of a portion of the telescopeof FIGURE 1 when modified with a still further type of image rotatingmechanism.

FIGURE 1l is a side view of the telescope of FIG- URE 10.

FIGURE 12 is a plot of the output voltages developed by thephoto-sensing means of FIGURE l when the scanning device is utilized inaccordance with the invention.

FIGURE 13 shows the phasing of the signal output of FIGURE l2.

FIGURE 14 is a line diagram which schematically illustrates theelectrical circuitry for controlling an azimuth servo motor and altitudeservo motor in accordance with the present invention.

Referring now to FIGURE l, I have shown a star tracking system whichincludes a telescope housing 2) which has an objective lens systemschematically shown by lens 21. Lens 21 focuses the light 21a from somecelestial body at which the housing 2) is pointed on a focal plane 22shown in dotted lines.

A scanning mechanism 23 to be described more fully hereinafter iscontained within telescope housing 2G so that a scanning aperture movesin plane 22. After mod.- ulation of the light by the scanning mechanism23, the light impinges upon a focusing system schematically shown bylens 24, and is thereafter focused upon Some type of photo-sensitiveelement 25 which could, for example, be a photo-multiplier tube.

The output signals generated by photo-sensing device 25 are then appliedto amplifier 26 which is connected to a servomechanism 27 which operatesto control the alignment of telescope 2) to cause telescope housing 2t)to follow the celestial body being tracked.

The scanning mechanism 23 can be' of any desired type which causes anaperture to move with simple harlmonic motion through the image of thecelestial body produced at focal plane22 by objective 2.1.

A typical scanning mechanism is shown in FIGURES 3 and 4 which comprisesa thin read 28 of magnetic material which is rigidly secured to a base29 (which could be telescope housing 20 of FIGURE l), and has anaperture plate 30 at its opposite end. The aperture plate 39 has anaperture 31 therein whose diameter is approximately equal l to thediameter of thc image ofthe celestial body in plane 22.

A solenoid means which includes -a core 32 and solenoid winding 33 whichistconnectcd to an A.C. source 34 will cause reed 2S to vibrate withsimple harmonic motion at the frequency of the A.C. source 34. Theexcursion of this travel is preferably such that aperture 51 movesapproximately four star images. That is to say, the aperture 31 will becaused to scan an area which is approximately four star images long andone star image wide.

The manner in which such scanning will give an indication of therelative alignment between telescope housing 20 and the celestial bodybeing tracked is fully described in above noted copending applicationSerial No. 71,248. As shown in that application, a single scanningmechanism such as the reed of FIGURES 3 and 4 will deliver informationonly as to one of an azimuth or altitude axis.

This operation is such that when the star image is focused at a nullposition (the rest position of aperture 31) and as the aperture iscaused to vibrate, the radiation from the celestial body will beinterrupted twice during each cycle of the reed. This will cause aperiodic signal to be developed by light-sensing device 25.

The fundamental component of this signal is equal to twice the reedfrequency or the frequency of source 34, and is shown, for example, inFIGURE 12 as curve 40. More specifically, curve of FIGURE l2 shows theamplitude of the second harmonie (232,) as a function of the imageposition for a constant image intensity. This signal is used to indicatethat the celestial body is lined up precisely with the telescope axis.

If the star is now moved off the null position and along the line ofvibration of aperture 31, then a periodic signal having a fundamentalcomponent equal to the reed frequency will be developed, as shown bycurve 41 of FIG- URE l2. This fundamental component will graduallyincrease in amplitude from zero to some maximum, and then decrease againas the star departs further from null.

If the star image moves olf null in a direction opposite to thatdescribed above, the amplitude variation will be as before as shown incurve 42 in FIGURE 12, but the phase of the fundamental component willreverse. The phase relationship of the outputs of curves 41 and 42 ofFIGURE 12 is shown in FIGURE 13 which shows phase cn the vertical axis,as compared to image position on the horizontal axis plotted in aperturediameters.

Thus, the signal generated by photo-sensing device 25 can be utilized toservo the telescope 29 in azimuth or altitude depending upon the linealong which aperture 31 oscillates. That is to say, normally when asingle reed such as reed 28 of FIGURE 3 is used, only a single axis isestablished, and it has been necessary in the past to provide stillanother scanning mechanism in which an aperture moves in a directionperpendicular to aperture 31 to establish the other axis. This, however,requires the provision of a further scanning mechanism which introducesadditional continuously moving elements into the system and thusprovides additional possible source of failure for the system.Furthermore, the weight of the total system is increased.

I have recognized that I can produce the effect of two scanningmechanisms by causing the image produced by the telescope objective tobe rotated by some angle, preferably 90, whereby the single scanningmechanism will first scan along a rst axis with respect to the image,but, when the image is rotated, will scan a second axis with respect tothe image. Thus, instead of requiring two complete scanning mechanisms,I can now provide only a single scanning mechanism and thus only one setof continuousry oscillating elements, and replace the other completescanning mechanism that would be normally required by a relativelysimple image rotating means.

As an illustration of an image rotating means which can be used inaccordance with the present invention, I show in FIGURES 1 and 2 themanner in which a Dove prism Si) may be interposed between the telescopeobjective 21 and the incident radiation 21a.

The Dove prism 59 is secured within a plate 51 which rides in an annularbearing 52 secured to telescope housing 26 so that plate 51 and the Doveprism 50 secured thereto are rotatable with respect to the telescopehousing 20.

A tirst and second projecting pin 53 and SJ. respectively are providedfor plate 51 where the pin 53 receives one end of a biasing spring 54awhich has its other end secured in any desired manner to housing 2S,while pin 54 receives a link 55 (FIGURE 2) of a solenoid operatingmechanism. More specifically', link 55 extends through a slot 56 intelescope housing 20 and is pivotally connected by pivot pin 57 tosolenoid plunger 58. The solenoid plunger 58 is contained within asolenoid coil schematically shown as coil 59 which is secured to theexternal portions of housing 2G.

The aforementioned mechanism is so designed that when the solenoid coil59 is not energized, the plate 51 assumes the position shown under theinfluence of biasing spring 54a. When, however, the solenoid 59 isenergized, the plunger 58 is retracted into the solenoid and thus causeslink 55 to exert a force on pin 54 to rotate plate 51 in acounterclockwise direction in FIGURE 2 against the biasing force ofspring 54a. This rotation is made to be so that, as will be seenhereinafter, there will be an image rotation of 90. Once the solenoid 59is again deenergized, the spring 54a will then be able to rotate plate51 back to the position shown. If desired, a rotary solenoid could beused in place of the operating mechanism described above to directlyrotate the prism.

The aforementioned control of the angular relationship of Dove prismwith respect to telescope housing 20 will cause a rotation of the imagefocused in focal plane 22 of objective system 21. That is to say, a Doveprism will operate so as to invert the vertical component of light rayspassing therethrough, although it leaves unaffected the horizontalcomponent of light rays passing therethrough. By horizontal andvertical, I mean here horizontal and vertical with respect to the bottomsurface of the prism, as shown in FIGURE 1. Thus, for example, if avertical arrow pointing upwardly were on the left of prism 50, the arrowwould be inverted when observed from the right of prism 5). If now theo-bserver and the arrow retain their rel-ative` positions and prism '50is rotated about the axis of telescope by 45 then the arrow observed bythe observer on the right of the prism would appear to rotate by 90.That is to say, only that component of the direction of the arrow whichis perpendicular to the base of prism 5t) is inverted. The Iarrow asseen by 4the observer then would be the resultant of this invertedcomponent of the arrow plus that component which is parallel to the baseof the prism which is not inverted, this resultant arrow being rotatedby 90 with respect .to the observed position of the arrow to the rightof the prism prior to rotation of the prism. The result, therefore, isthat a rotation of prism 50 caused by operation of solenoid 59 willcause a rotation of the image` of the celestial body observed bytelescope Ztl by twice the angle of prism rotation.

Accordingly, the scanning mechanism 23 which previously was scanning,for example, in azimuth will now be scanning the image in a directionperpendicular to this original azimuth direction, and thus will bescanning in altitude.

It will be noted that the use of a Dove prims 50 as described in FIGURES1 and 2 is only one manner in which the image opera-ted upon by scanningmechanism 23 can be rotated. By way of example, and as illustrated vinFIGURES 5 through 9, a pair of right angle mirrors can be providedwhere, as shown -in FIGURE 5, a first right angle mirror 60 is securedto telescope housing 20 in front of objective lens 21, while a rotatableright angle mirror 61 is pos-itioned in spaced relationship with respectto mirror 60.

A central portion of mirror 61 has ashaft 62 attached thereto whichextends through housing 20, and terminates in a crank arm 63, as shownin FIGURES 5, 6 and 7. Upon rotation of crank arm 63, the mirror 61 canbe rotated by 45 to the position shown in FIGURE 8.

The upper half of the opening of telescope housing 20 is normallyshielded by a semicircular shield 64, so that light is impinged onlyupon the left-hand phase of mirror 60. This light is then, by multiplereiiection, transmitted ultimately to the lower portion of Objectivelens 21. When the mirrors are in the relationship shown in FIG- URE 5,then the image will pass directly through the system and will not beinverted. This is schematically shown in FIGURE 9 where the light ray isillustrated by the dotted line so as to be reiiected from the upperleft-hand portion of mirror 60 to the inner left-hand portion of-rnirror 61 across to the inner right-hand portion of mirror 61 down tothe upper right-hand por-tion of mirror 60, and thence into theobjective 21. If now the mirror 61 is rotated as by 45 the imagetransmitted through objective lens 21 will be rotated through 90 so thatthe single scanning mechanism provided as shown in FIG- URE 1 can scanalternately in azimuth and then in altitude.

The operating mechanism for driving mirror 61 can include, for example,a solenoid mechanism schematically illustrated as solenoid mechanism 65.Solenoid mechanism 65 has an output link 66 (FIGURE 6) connected to theend of crank arm 63 and moves crank arm 63 against the force of abiasing spring 67 secured to housing portion 68 connected to housing 2%)which normally biases the mirror toward some predetermined position suchas the position shown in FIGURE 5. It is to be noted that the mirror 61can be housed in an extension of housing 20 so that the surface ofmirror 60 extends completely across the telescope housing opening. Withthis modification, the whole telescope objective aperture is utilized.

As a still further embodiment of the invention, and as is shown 4inFIGURES 10 and l1, two relatively small Dove prisms 70 and 71 may besupported from housing 29 by pedestals 72 and 73 respectively in themanner shown in FIGURES 10 and 11. The prisms, as best shown in FIGURE11, are arranged at a 45 angle with respect to one another, and arepositioned in `front of a disk 74 which is contained within an annularbearing groove in ring 75 which is secured to housing 20. Disk 74 has anaperture 76 therein which, by rotation of disk 74, may be positionedbetween prism 70 and objective lens 21 or between prism 71 and objectivelens 21. In the position of FIGURES 10 and 11, the aperture 76 is inalignment with prism 70.

When the aperture 76 is in alignment with prism 70, the image formed byobjective lens 21 will have a first predetermined alignment. When,however, disk 74 is rotated -so as to bring aperture 76 into alignmentwith prism 71 and to obscure prism 70, the resultant image will rotateby 90 since prisms 70 and 71 are at 45 angles with respect to oneanother. Thus, the scanning means contained at the focal plane ofobjective lens 21 can scan alternately in azimuth or altitude.

In order to rotate disk 74, any desired mechanism such as a rotarysolenoid may be provided. By way of example, and as shown in FIGURES 10and ll, a motor 77 secured to housing 20 may have an output drivingwheel 7S which has some frictional type engaging material on its outersurface thereof such as rubber which bears against the surface of disk74. The disk 74 is further provided with a projecting pin 79 which movesbetween projecting posts 80 and 81 respectively whereby when motor 77 isdriven in a first direction so that disk 74 rotates in a clockwisedirection in FIGURE l1, the disk 74 rotates until pin 79 engages stop80, the motor thereafter supplying only enough torque to retain disk 74in this position. The image then being scanned is that passed throughprism 7G.

When it i-s now desired to scan in a different direction, the energizingcircuit to motor 77 is controlled so as to reverse the direction ofrotation of motor 77 in any appropriate manner whereby the disk 74 isrotated from the position of FIGURE 11 in a counterclockwise directionuntil post 79 engages stop 81. Thereafter, the motor 77 need only supplyenough torque to retain this new position. In this position, theaperture 76 of disk 74 is rotated into alignment with prism 71 'so thatthe image being scanned is rotated by A typical type of electricalsystem for use with the novel scanning mechanism of the presentinvention is schematically illustrated in FIGURE 14. Referring now toFIG- URE 14, an input voltage which typically may be at 400 cycles isconnected to terminals 90, 91 and 92 where at terminal 90 the voltage isapplied to the driving coil 33 of the reed scanner schematicallyillustrated as including the reed 28 having aperture plate 30 at one endthereof, while terminals 91 and 92 are connected to fixed iield windings93 and 94 of altitude control motor 95 and azimuth control motor 96Irespectively of the servo mechanism 27 of FIGURE 1. An image positioncontrol means 97 is then schematically illustrated as being positionedin front of the aperture 31 and in line with the light-sensing means 25shown in FIGURE 14 as a photomultiplier which could be of the type 1P21.

The control means 98 for controlling the operation of image controlmeans 97 is schematically illustrated for image control means 97, andcould, for example, be the motor or solenoid control previouslydescribed above in the various embodiments of the invention.

Control means 98 is then electrically connected to switching means 99and 100 respectively where, when control means 98 calls for, forexample, the azimuth mode of operation, it will open the switching means99 alegan/s and close switching nte-ans lh?. Alternatively, when controlmeans 9S calls for altitude mode of operation, it closes switching means99 and open switching means 169.

The output of photo-multiplier 25 is connected to SG()- cycle tunedamplicrs 1G31 and 162 and 40G-cycle tuned amplifiers 163 and 184 throughthe switching means 160 and 99 respectively. That is to say, during thealtitude mode of operation, the switching means 99 is closed andswitching means 18S is open so that the output of photosensing device 25is connected to amplifiers 102 and 164. During azimuth mode ofoperation, however, only switching means 166 is closed so that theoutput of photo-sensing device 25 is connected only to ampiers 161 andi523.

The outputs of each of amplifiers 101 through 164 are then connected toacquisition control circuits 165, 196, 107 and 168 respectively whichhave outputs connected in any desired manner to drive the servomechanism elements in order to maintain the 86C-cycle double frequencyoutput of photo-sensing device 25. This, of course, will be the outputfrequency of photo-sensing device 25 when the star image is at a nullposition or central position in azimuth and in altitude, since reed 22S-is oscillate-:l at a frequency of 400 cycles.

If, during the altitude mode of operation, the star image moves off itsnull position, a signal will be received in the 40G-cycle amplifier 104.Amplifier 1%4 is connected to the control field winding 103 of servomotor 95 whereby the motor 95 is energized responsive to an excursion ofthe celestial body image from a null position, the phase of theenergization being dependent upon the sense of the excursion, asillustrated above.

In an identical manner, and during scanning in the azimuth mode, adeviation of the celestial body image from null will cause a signal tobe developed in the 400- cycle amplifier 163 to deliver a signal tocontrol field winding 199 of servo motor 96 which will cause therepositioning of the telescope housing to reduce the signal to null.

Accordingly, both the azimuth control motors 96 and altitude controlmotor 95 will operate to reposition the azimuth and altitude of thetelescope to maintain the proper telescope alignment for retaining thecelestial body image at null.

It is to be noted that, while in FIGURE 14 I have shown independentamplifiers for both the azimuth and altitude channels, a commonamplifier channel could be used with appropriate switching mechanismsbetween the amplifiers and their respective acquisition circuits whichare operable from control device 98.

Thus, it can be seen from FGURE 14 that the novel invention, in additionto permitting simplification of the scanning mechanism, can furtherpermit simplification in the electrical circuitry used in the trackingsystem.

Although I have described preferred embodiments of my novel invention,many variations and modifications will now be obvious to those skilledin the art, and I prefer, therefore, to be limited not by the specificdisclosure herein but only by the appended claims.

I claim:

1. A light source tracking system comprising telescope means for imagingthe light of a light source to be tracked in a focal plane, a scanningmeans, a light sensing means and an image rotating means; said scanningmeans including a vibrating aperture movable in the plane of said focalplane and vibrating along a 4substantially straight line with simpleharmonic motion; said vibrating aperture having a null position alongthe optical axis of said telescope means; said light sensing means beingpositioned along the optical axis of said telescope means; saidvibrating aperture being interposed between said telescope means andsaid light sensing means; said image rotating means being positioned infront of said telescope means and said scanning means; said imagerotating means being rotatable from a first fixed position to a secondfixed position to rotate said image in said focal plane through apredetermined angle.

The system substantially as set forth in claim 1 'wherein said imagerotating means rotates said image 3. The system substantially as setforth in claim 1 wherein said image rotating means comprises a Doveprism.

4. The system substantially as set forth in claim 1; said image rotatingmeans being comprised of a pair of right angl: mirrors positioned inspaced relation with respect to one another for transmitting light fromsaid source; to said focal plane by multiple retiection; one of saidmirrors being' rotatable to rotate said image.

References Cited by the Examiner UNITED STATES PATENTS 2,406,798 9/46Burroughs 88-1 2,553,171 5/51 Campos 88--61 2,917,967 l2/59 Steglich 8812,923,202 2/60 Trimble 88-1 2,928,952 3/60 Bednorz 88-1 2,939,962 6/60Miller 88--1 2,947,872 8/60 Carbonara et al. 88--1 3,302,098 9/61Watkins 88--1 3,057,953 l0/62 Guerth 88-1 X 3,061,730 itl/'62 JankowitzZ50-203 3,334,261 4/63 Wilson 88--1 .TEWE` L H. PEDERSEN, PrimaryExaminer.

EJL G. ANDERSON, Examiner.

1. A LIGHT SOURCE TRACKING SYSTEM COMPRISING TELESCOPE MEANS FOR IMAGINGTHE LIGHT OF A LIGHT SOURCE TO BE TRACKED IN A FOCAL PLANE, A SCANNINGMEANS, A LIGHT SENSING MEANS AND AN IMAGE ROTATING MEANS; SAID SCANNINGMEANS INCLUDING A VIBRATING APERTURE MOVABLE IN THE PLANE OF SAID FOCALPLANE AND VIBRATING ALONG A SUBSTANTIALLY STRAIGHT LINE WITH SIMPLEHARMONIC MOTION; SAID VIBRATING APERTURE HAVING A NULL POSITION ALONGTHE OPTICAL AXIS OF SAID TELESCOPE MEANS; SAID LIGHT SENSING MEANS BEINGPOSITIONED ALONG THE OPTICAL AXIS OF SAID TELESCOPE MEANS; SAIDVIBRATING APERTURE BEING INTERPOSED BETWEEN SAID TELESCOPE MEANS ANDSAID LIGHT SENSING MEANS; SAID IMAGE ROTATING MEANS BEING POSITIONED INFRONT OF SAID TELESCOPE MEANS AND SAID SCANNING MEANS; SAID IMAGEROTATING MEANS BEING ROTATABLE FROM A FIRST FIXED POSITION TO A SECONDFIXED POSITION TO ROTATE SAID IMAGE IN SAID FOCAL PLANE THROUGH APREDETERMINED ANGLE.